Seatback frame



FIG. 1 is a left front perspective view of a seatback frame showing my new design;

FIG. 2 is a side elevational view, the opposite side being a mirror image thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is an top view thereof; and,

FIG. 5 is a left front perspective view of my design, shown in a representative position with environmental elements.

The views of the seatback frame not shown form no part of the claimed design. The dashed lines in FIG. 5 represent environmental elements only and form no part of the claimed design. 

The ornamental design for a seatback frame, as shown and described. 